Conventionally, there has been proposed a charged-particle beam apparatus for emitting a charged-particle beam onto a sample in a vacuum atmosphere to fabricate or observe a surface of the sample. FIG. 3 schematically shows a charged-particle beam apparatus 50 as described above. The charged-particle beam apparatus 50 includes a chamber 51 in which a sample S is placed, and a lens-barrel 52 for directing a charged-particle beam B onto the sample S placed in the chamber 51. The lens-barrel 52 includes a charged-particle supplier 53 having a charged-particle source 53a, a condenser lens 54 and an objective lens 55 each serving as an electrostatic lens to which a voltage can be applied. Moreover, the chamber 51 is provided with an evacuating means 56 capable of evacuating the chamber 51, and a gas supplier 57 capable of supplying a gas such as N2 into the chamber 51. In the charged-particle beam apparatus 50 described above, a voltage is applied to the objective lens 55 in such a manner that the evacuating means 56 creates a high vacuum atmosphere inside the chamber 51 and the lens-barrel 52. In this state, a charged particle drawn from the charged-particle source 53a of the charged-particle supplier 53 is accelerated, and then is emitted as a charged-particle beam B50. Then, the released charged-particle beam 50B is converged by electric fields formed by the condenser lens 54 and the objective lens 55, respectively, and is irradiated onto the sample S, so that a surface of the sample S is subjected to fabrication or observation. As described above, after the completion of fabrication or observation of the sample S, the sample S is transported to the outside through a sample transport opening (not shown). Herein, in each of the interior of the chamber 51 and the interior of the lens-barrel 52, a high vacuum atmosphere is created. Consequently, when the sample transport opening is open, a large amount of outside dust disadvantageously enters each interior in addition to outside air. In order to avoid such a disadvantage, the chamber 51 and the lens-barrel 52 are filled with a gas by the gas supplying means 57 at a nearly atmospheric pressure, and then the sample S is transported to the outside through the sample transport opening
As an example of the charged-particle beam apparatus 50 described above, more specifically, there is a focused ion beam apparatus that emits an ion beam to a sample with the use of an ion source serving as a charged-particle source (refer to, e.g., Japanese Unexamined Patent Publication No. 2002-251976). The focused ion beam apparatus 50 described above has an advantage that an ion beam is emitted to a sample in order to etch a surface of the sample or an assist gas is supplied to the sample simultaneously with the emission of the ion beam, so that a deposition is formed on the surface of the sample. As another example, moreover, there is a scanning electron microscope that emits an electron beam, which has been drawn from an electron source serving as a charged-particle source and then has been accelerated, to a sample to observe a surface of the sample. With regard to these charged-particle beams, when a high voltage is applied to an electrostatic lens used as an objective lens, optical aberration becomes small. As a result, the charged-particle beam can be converged effectively. When the charged-particle beam is converged effectively as described above, the focused ion beam apparatus can fabricate the surface of the sample more precisely. On the other hand, the scanning electron microscope can obtain an observation image at a high resolution. Recently, a sample to be fabricated or observed tends to become finer. In order to fabricate and observe such a sample more precisely, therefore, there has been proposed a method of applying a higher voltage to an electrostatic lens to fabricate or observe the sample.
However, the application of a higher voltage to the electrostatic lens causes the following problem. That is, when dust is attached to the electrostatic lens, even if the attached dust is in such a small amount as to not pose any problem in the conventional art, electric discharge occurs at the electrostatic lens. As described above, conventionally, when a sample is transported from the chamber to the outside, the gas supplying means supplies a gas to the chamber to prevent outside dust from entering the chamber. However, it is impossible to prevent the dust from entering completely. In addition, since dust is also attached to the sample, a small amount of dust always exists in the chamber. When the gas supplying means supplies the gas into the chamber to create a high vacuum atmosphere, such dust is stirred up together with the gas, and then is flown into the lens-barrel. Herein, the dust is attached to the objective lens, resulting in electric discharge.
The present invention has been devised in view of the circumstances described above. An object of the present invention is to provide a charged-particle beam apparatus capable of preventing a small amount of dust from being attached to an electrostatic lens serving as an objective lens to apply a high voltage to the electrostatic lens.